1. Field of the Invention
The present invention relates to titanium discs which are useful for the production of high-density read-while-write magnetic discs.
2. Description of the Related Art
Aluminum discs have been widely used to produce magnetic discs. However, there has been an increasing need to increase the recording density of magnetic discs and reduce its thickness. To meet these requirements, attempts have been made to use titanium, which has excellent heat resistance and rigidity, as a new material for magnetic discs in the place of aluminum. See Japanese Patent Application Laid Open No. 105804/1977. However, the uneven crystalline surface of polycrystalline titanium is reflected through the magnetic layer provided as the recording medium, and causes frequent signal errors.
The inventors of the present invention have found an effective means to prevent such errors by coating the disc surface with an amorphous anodic oxidation layer. However, evaluation of the properties of the titanium magnetic discs coated with an amorphous anodic oxidation layer shows that there is insufficient stability when texturing the anodic oxidation layer. This in turn causes inconsistency in the "Contact Start Stop" property, also known as the CSS property, of such discs. The CSS property is the variation of the friction between a magnetic head and a disc when the head is on and off the disc.
Texturing is a process by which the surface of a disc is roughened to form minute irregularities as concentric circles in the direction of the diameter of the disc. Texturing is used to overcome the problem of contamination of the disc surface by water and/or lubricant absorbed on the magnetic head when it is caused to stop on the magnetic disc surface. This problem arises because the surfaces of the thin layered magnetic discs, such as sputtered or plated discs, are much finer than conventional magnetic discs, and is easily marred by contamination.
One method of providing such irregularities is by tape texturing wherein the minute irregularities are formed, for an example, by spinning the disc at 50 rpm while a 2 inches wide polyethylene tape provided with abrasive particles of white alumina having an average diameter of 6 .mu.m is pressed onto the disc surface and is moved in the radial direction of the disc at 800 mm/min.
It is, therefore, important to provide a surface that can be stably textured. This has been a major obstacle in producing titanium magnetic discs.
Non-electrolytic plating with Ni-P is well known to be effective for surface treating aluminum alloy discs and has been used widely on an industrial scale. Such nonelectrolytic plating provides a variety of properties to the aluminum alloy discs. See Yoshio Hirayama: Metal Surface Technology, Vol. 38, p. 378/1987. Texturing of the nonelectrolytic Ni-P plated layer on the aluminum alloy disc is also well established, from which satisfactory CSS property has been obtained.
The thickness of a Ni-P plated layer on an aluminum alloy disc should be between 15 and 20 .mu.m, of which approximately 5 .mu.m is removed by polishing to improve the surface roughness. See Japanese Patent Application Laid Open No. 222433/1987. As a result, a plated layer with a thickness of 10 to 15 .mu.m on the finished product is provided. This thickness is required to provide sufficient rigidity to the comparatively soft aluminum alloy discs and at the same time achieve satisfactory CSS property. However, it takes approximately 120 minutes to form a Ni-P plated layer with a thickness of 15.about.20 .mu.m. Moreover, when a Ni-P plated layer of 15.about.20 .mu.m is formed, the surface roughness represented by R.sub.a, as specified in JIS B0601, may reach as high as approximately 0.5 .mu.m. It is necessary, therefore, to polish the surface of the disc after it is plated with Ni-P as reported in Japanese Patent Application Laid Open No. 222433/1987. The time taken to plate the aluminum disc and the subsequent polishing required have been the major obstacles in producing inexpensive aluminum alloy discs. See Tetsuo Otaka et al., Metal Surface Technology, Vol. 38, p. 554 (1987).
Here, R.sub.a is a measure of the roughness of a surface and is defined as the mean height of the peaks from an imaginary center line, where roughness is represented by peaks and valleys on a surface. It is represented by the equation: ##EQU1##
l: is a standard length of about 250 .mu.m to 800 .mu.m and is sufficiently large to include peaks and valleys so that R.sub.a represents a surface roughness.
The center line is so drawn that the area covered by the peaks above the line and area covered by the valleys under the line is equal. The equation for the center line is as follows: ##EQU2##
When R.sub.a is as high as 0.5 .mu.m, a final polishing is required. However, this final polishing causes defects and reduces the quality of the finished product.
It would be desirable to provide a titanium disc having the desirable properties to be used as a base disc, and a titanium disc having a non-magnetic plated layer thereon. A titanium base disc useful to produce a magnetic disc according to the present invention is to have a desirable surface roughness so that the titanium disc can be plated with a highly adhesive plated layer. It is also desirable to provide a titanium disc useful as a magnetic disc having sufficient thickness of the plated layer to allow texturing of its surface.
However, titanium is known to be difficult to plate, and attempts to form a strongly adhesive plated layer on the mirror-like surface of the titanium disc have not been successful. An effective means to form a highly adhesive plated layer is to roughen the surface of the titanium. See the 79th Seminar Report of the Surface Technology Association p. 230 (1989). However, the increased surface roughness makes re-polishing of the plated layer inevitable. Therefore, this process has been inappropriate for production on an industrial scale.
Therefore, the first object of the present invention is to provide a titanium disc on which a highly adhesive plated layer can be formed.
The second object of the present invention is to provide a magnetic disc having a plated layer with an appropriate thickness. This avoids the increased residual stress caused by depositing the magnetic layer by sputtering, as the thickness of the plated layer increases. The residual stress in the plated layer causes exfoliation of the plated layer. Further, excessive thickness of the plated layer also results in inferior surface roughness which in turn requires surface finishing by polishing after the disc is plated.
Thus, a further object of the present invention is to provide a titanium disc with a desirable plated layer.
The present invention provides a titanium disc having a plated layer which does not exfoliate when a magnetic layer is formed by a sputtering process. Moreover, the process provides a minimum thickness of the plated layer such that the surface of the titanium disc is not exposed when texturing is done on the plated layer.